The fact that fully synthetic oil has longer lasting qualities is already stated in other responses here. BMW now approve the use of fully synthetic ON THE LATEST ENGINES and have been able to increase service intervals as a result. BMW UK do NOT recommend anything better than semi-synthetic in earlier engines. My M50TU is run on Castrol Magnatec 10w/40, changed at 5,000 miles (6 months). No need to spend more - you'll gain no benefit.